Fixup 41caf2d for DeltaValue tables

Not that I can find any fonts having these... Had to construct one
to test.
diff --git a/Lib/fontTools/ttLib/tables/otBase.py b/Lib/fontTools/ttLib/tables/otBase.py
index e6fd0ab..43123f1 100644
--- a/Lib/fontTools/ttLib/tables/otBase.py
+++ b/Lib/fontTools/ttLib/tables/otBase.py
@@ -555,7 +555,7 @@
 
 			else:
 				table[conv.name] = conv.read(reader, font, countVars)
-				if conv.isCount:
+				if conv.isCount or conv.isSize:
 					counts.append(conv.name)
 					countVars[conv.name] = table[conv.name]
 
@@ -618,6 +618,9 @@
 				countVars[name] = storeValueFactory(table, name)
 			else:
 				conv.write(writer, font, countVars, value)
+				if conv.isSize:
+					countVars[conv.name] = value
+					counts.append(conv.name)
 
 		for count in counts:
 			del countVars[count]